Materials for packaging foods and various articles are often required to have gas barrier properties, particularly, oxygen barrier properties. This is intended to prevent effects such as oxidation degradation of packaged contents due to oxygen, for example. In particular, with regard to food packages, the presence of oxygen allows microorganisms to proliferate and thereby the contents decay, which is a problem. Therefore, in conventional packaging materials, gas barrier layers for preventing oxygen from passing therethrough are provided, so that the permeation of oxygen, etc. is prevented.
Such a gas barrier layer can be, for example, metal foil, or a vapor deposition layer of metal or a metal compound. Generally, aluminum foil, an aluminum vapor deposition layer, a silicon oxide vapor deposition layer, an aluminum oxide vapor deposition layer, and the like are used. However, metal layers such as an aluminum foil and an aluminum vapor deposition layer have disadvantages, such as the invisibility of packaged contents and the difficulty of disposal. Furthermore, metal compound layers such as a silicon oxide vapor deposition layer and an aluminum oxide vapor deposition layer have disadvantages of considerable degradation of the gas barrier properties caused by deformation or dropping of the packaging material, or impact during transportation, for example.
Moreover, a layer formed of a vinyl alcohol polymer with excellent gas barrier properties, such as polyvinyl alcohol and ethylene-vinyl alcohol copolymer, may be used as a gas barrier layer in some cases. A layer formed of such a vinyl alcohol polymer has advantages of the transparency and less difficulty in disposal. Accordingly, the range of applications thereof is now increasing.
The above-mentioned vinyl alcohol polymer is crystallized by hydrogen bonds between the hydroxyl groups in the molecules, and thus exhibits gas barrier properties. Hence, the conventional vinyl alcohol polymer exhibits high gas barrier properties in a dry state. However, in a state where the polymer has absorbed moisture under the influence, for instance, of water vapor, the hydrogen bonds are weakened and thereby the gas barrier properties tend to deteriorate. Accordingly, it is difficult to allow a vinyl alcohol polymer such as polyvinyl alcohol to exhibit a high level of gas barrier properties under high humidity.
Further, materials containing a polymer compound and the hydrolyzed condensate of metal alkoxide (for instance, tetramethoxysilane) have been studied as a material with gas barrier properties (for example, JP 2002-326303 A, JP 7 (1995)-118543 A, and JP 2000-233478 A).
Moreover, a material formed of polyacrylic acid and a crosslinking component has been studied as a material with gas barrier properties (for example, JP 2001-310425 A).
Recently, retort foods, which are produced by packing contents in a food packaging material and then immersing it in hot water for a sterilization treatment, have become widespread. In such a situation, the required level of performance of packaging materials for retort foods, such as strength against bag-breakage at the time of dropping a food packaging material filled with contents, oxygen barrier properties after hot water sterilization, and oxygen barrier properties under high humidity during the delivery to a consumer, is increasing further. In particular, there is demand for packaging materials capable of achieving high oxygen barrier properties regardless of humidity and achieving high oxygen barrier properties even after being subjected to retorting, as well as having excellent strength and transparency. The above-mentioned conventional techniques, however, cannot satisfy such demand sufficiently.
As a result of studies to solve the above-mentioned problems, the inventors have found a method that enables the properties of a gas barrier layer to be improved dramatically (WO 2005/053954 A1). In this method, a gas barrier layer formed of a composition that includes the hydrolyzed condensate of metal alkoxide and a polymer containing a —COO— group is immersed in a solution that contains a metal ion having a valence of at least two. The —COO— group in the polymer is neutralized by this treatment.
According to the method of WO 2005/053954 A1, the properties of the gas barrier layer can be improved dramatically. However, the packaging materials to be used for retort pouch, for example, are required to withstand treatment under severe conditions, and thus desirably have higher properties. A reduction in thickness of the gas barrier layer is also required in order to enhance the dimensional stability during processing, such as printing and lamination, and the flexibility of a gas barrier layered product itself, so that the mechanical properties of the gas barrier layered product should be close to the original mechanical properties of a base film. However, when the gas barrier layer has a reduced thickness, its oxygen barrier properties deteriorate considerably in some cases. Further, the conventional gas barrier layers had a decrease in transparency in some cases when being subjected to retorting under severe conditions.